The present invention relates to electrical safety devices and, more particularly, to overtemperature and mechanical damage sensors for electrical wire, cable, cords and appliances.
Historically, electrical apparatus have depended on overcurrent devices such as circuit breakers and fuses to protect the apparatus from overtemperature conditions which might lead to equipment damage or personnel injury resulting from fire or smoke. Overcurrent devices suffer from limitations in their effectiveness, in that overtemperature conditions can result from normal current flow through equipment suffering from damaged conductors or poor electrical connections. For example, an electrical cord containing stranded wire conductors may overheat if some of the strands are broken, even if less than rated current flows through the cord. Many fires have been caused due to poor connections in plugs, receptacles, and connection strips in cords and appliances.
Ground fault interrupters (GFI) are another common safety device used in electrical equipment. While such devices are effective in reducing electrical shock and equipment damage due to shorts to ground, they are ineffective in cases where the fault does not result in current flow to ground. For example, a GFI will not prevent a fire in the case of broken conductor strands or poor connections because no ground current flow occurs. Neither GFIs nor overcurrent devices protect a cord or device from overtemperature conditions resulting from external sources of heat such as excessive ambient temperature conditions or contact with hot burners, hot piping, etc. Neither device may be effective if the cord is unable to dissipate normal heat. For example, the cord or appliance may become covered with thermally insulating material that prevents dissipation of heat due to normal current flow.
Temperature sensors such as resistance temperature sensors (RTDs) and thermistors are commonly used to detect overtemperature conditions in equipment. These devices may be used to initiate alarms or relays to interrupt current to the device upon overtemperature. While these devices are effective in sensing temperature in a small or enclosed space, they are ineffective in sensing overtemperature over a long distance or in a large volume unless many devices are employed. Use of sufficient devices for sensing temperature over a long distance or large volume raises the cost of protection substantially. Also, the wiring needed for connecting a large number of sensors complicates the device and increases the size and bulk of the device.
None of the devices above provides protection from mechanical damage to cords or equipment cases which may result in exposed conductors or internal shorts. None of the devices provides protection from shock or fire from damaged electrical cords or equipment until a sufficient fault current begins to flow. For example, a person using a faulty piece of equipment with ground fault interruption who establishes a ground path must experience a momentary shock in order for the interrupter to open the circuit.
None of the devices above provides protection from mechanical damage or overtemperature for a bundle of wires or cables.